Regular people may not but password managers do. Maybe not everyone, but at least security-conscious people would grow suspicious when their password manager do

Regular people may not but password managers do. Maybe not everyone, but at least security-conscious people would grow suspicious when their password manager doesn't autofill credentials on a page, and wouldn't immediately jump to manually entering it. (SSO also reduces the attack surface of phishing, though of course then the attacker just has to phish the identity provider's credentials instead.)